3-1-Visual observations of self-healing
To investigate the effect of self-healing and the extent to which the cracks are filled with bacterial precipitation, the cracked specimens were taken out of the tank at different ages and the healing of the cracks was determined. The mixtures in which the bacterial nutrient was added into the curing environment had a slower rate of self-healing. The rate of self-healing increased dramatically in Ni-2 and Cl-2 mixtures in the first 10 days of re-curing.
3-2-Compressive strength test
The compressive strength results of the mixture and the percentage of increase or decrease in the compressive strength compared to the control concrete are shown in Fig. 3. It is clear that how to use of bacteria and its nutrient play an important role in increasing or decreasing the compressive strength of concretes. When calcium nitrate-urea and calcium chloride-urea were added to Ni-2 and Cl-2 mixtures; compressive strength respectively increased by 22.22% and 9.54% after 7 days of curing and 23.43% and 7.5% after 28 days of curing compared to the control concrete at the same ages. Because the source of calcium is available to bacteria, the ability of bacteria to precipitate calcium carbonate is present from the beginning, which increases the compressive strength. This process was also seen in the concretes containing air content. However, other mixtures showed a decrease in the compressive strength compared to the control concrete. Although, the air content in the concrete itself reduces the compressive strength, but when bacteria and bacterial nutrient were used in the concretes containing 5% air voids, this reduction in strength is partially compensated. HNi-2 had a 7.5% increase in the compressive strength after 7 days of curing compared to the control concrete. Obviously, if the bacterial nutrient is used in curing environment, calcium carbonate precipitation is very slow, but it is expected that the performance of this type of curing will be evident in the self-healing process. Moreover, the performance of calcium nitrate in all mixtures was better than that of calcium chloride. Fig. 4 shows the relative increase in the compressive strength of the mixtures containing bacteria nutrient in comparison with the concretes that the bacteria nutrients were added to the curing water after 7 days and 28 days curing. In comparison to the control concrete, the increase in the compressive strength of Ni-2 and Cl-2 mixtures after 7 days of curing was more than 28 days of curing. However, a decrease in the compressive strength was observed for other mixtures in comparison to the control one at the age of 7 days in comparison with 28 days. In fact, it can be inferred that the activity of bacteria is higher at the younger ages.
The effect of adding and separating culture medium on the compressive strength of the relevant bacterial mixtures after 7 days and 28 days curing is shown in Fig. 5. According to Fig. 5, it can be pointed out that centrifugation of bacteria significantly reduces the compressive strength and it is better to use the bacteria with the culture medium in concretes.
3-2-Tensile strength test
Tensile strength test results of the concretes are shown in Fig. 6-a. As shown, all mixtures even concrete containing air-entrained agent, have higher tensile strength than control concrete, which can be due to the calcium carbonate precipitation. Therefore, it can be concluded that entrained airs in bacterial concrete can increase tensile strength despite reduction in the compressive strength. According to Fig. 6-b which shows the relative increase in tensile strength compared to control concrete, the highest tensile strength was related to Ni-2 mixture with 20.56% increase in tensile strength. Also, similar discussions to compressive strength can be drawn for the tensile strength variations.
3-4-Secondary tensile strength (STS) test
The results of STS test after 5 weeks of re-curing and the ratio of secondary tensile strength to the relevant initial tensile strength are shown in Fig. 7-a. All mixtures be able to achieve relatively higher tensile strength than the control concrete, which indicates a mechanical improvement in cracked concretes, although they have not achieved their initial tensile strength. However, Cl-2, HCl-2, and Ni-2 mixtures gained the maximum secondary tensile strength, respectively where the bacterial nutrients were added to the mixtures. Fig. 7-b shows the relative increase in STS compared to the control concrete. The highest STS was related to Ni-2 and Cl-2 mixtures, which were associated with 109% and 104% increase in tensile strength compared to the control concrete, respectively, which is caused by the precipitation of calcium carbonate and the repair of cracks by bacteria. Since the cracks of the specimens were closed after two weeks of curing according to the water passing through the crack test, so further re-curing has made the structure of the repaired crack more compacted and the tensile strength has been significantly improved. This phenomenon has been delayed in other mixtures and has slowly increased the tensile strength. Also, HCl-2, HNi-2, SHCl-2, and SHNi-2 mixtures with 5% air voids had higher secondary tensile strength than HCl-1 and HNi-1 mixtures. Fig. 8-a shows how the mixture Ni-2 repaired the crack after 5 weeks of re-curing. On the other hands, its secondary failure under splitting test is presented in Fig. 8 b, which indicates the failure of the specimen exactly occurred from the healed crack.
3-5-Water passing through the cracked specimens
Water passing through the cracked specimens was measured after 0, 1, 2, 4, 6, 8, 12, 16, 18, 20, 22, 24, 26, 28, and 30 days of re-curing under the mentioned curing environments. All mixtures except the control mixture completely filled the cracks during 30 days of re-curing. In this section, mixtures were divided into two groups.
The first group includes the mixtures M, Cl-1, Ni-1, HCl-1, and HNi-1 where bacterial nutrient was added into the curing solution except the control mixture (M). This group had a slower rate of self-healing than other specimens, and the control mixture had the worst performance. The results of the first group are shown in Fig. 9-a where the amount of water passing through the crack in entrained air specimens was significantly reduced after 12 days of re-curing, and then was completely repaired after 20 days of re-curing. However, mixtures without entrained air were completely repaired after 30 days of re-curing. There was no significant difference between the effect of calcium nitrate and calcium chloride as a bacterial nutrient in the self-healing process, but generally, calcium nitrate appeared better. Since bacteria have more space in the entrained concretes, they can survive longer in the concretes and therefore their performance in the self-healing process will be better. It should be noted that when water does not penetrate during 6 hours through the repaired crack, it is assumed that the crack was completely repaired, but in fact, the crack was not perfectly repaired according to STS test results.
The second group includes Cl-2, Ni-2, HCl-2, HNi-2, SHCl-2, and SHNi-2 mixtures, where bacterial nutrient has been used in the concrete mixtures. The impact of nitrate calcium-urea on the self-healing process is more significant in comparison to the first group. According to Fig. 9-b, the cracks were completely repaired up to 14 days of re-curing and Ni-2 showed the best performance in which the crack was completely healed after 8 days of re-curing. Also, HCl-2, HNi-2, SHCl-2, and SHNi-2 mixtures did not differ much in terms of self-healing, however, when centrifuged bacteria and air-entraining agent were used, the self-healing process was slowly delayed.